阅读理解
   Betty is an American school girl. She is very happy because her parents will take her to China for a visit. They are going to fly from New York on June 30 and get to Beijing on July 1. They will stay at Lido Holiday Inn. Their Chinese friends will show them around Beijing. They are going to visit the Palace Museum, the Summer Palace, the Temple of Heaven and the Great Wall. They will also visit some Chinese homes and try some Chinese food. They will spend a week in Beijing and then go to Xi’an, Guilin and some other places for sightseeing. The whole trip will take three weeks and she is sure she will have a great time in China.

战国时期流通货币种类有刀、布、钱和爰金，刀币是齐国货币。考古发现在古代赵、燕及辽东和朝鲜北部亦有大量刀币流通，这一现象主要是因为（    ）

—No, it isn't. I think it's ____.